The influence of drastic dietary carbohydrate restriction on the occurrence of Strep. mutans, Strep. sanguis and iodophilic polysaccharide-producing bacteria in the dental plaque of 6 subjects was studied. During a carbohydrate-free period of 17 days, the percentage that Strep. mutans constituted of the total cultivable flora decreased to a very low or undetectable level, while simultaneously the percentage Strep. sanguis increased. The percentage IPS-producing bacteria was also lower during this period. Reinstitution of the normal diet, after periods of added glucose and sucrose to the carbohydrate-free diet, resulted in a return of Strep. sanguis to its original proportions and of Strep. mutans and the IPS-producers to levels that were significantly higher and slightly higher respectively than before the experiment. During the study an inverse relationship between the occurrence of Strep. mutans and Strep. sanguis was observed.